Prior to the reintroduction of coherent optical transmitters and receivers, a colourless Optical Add-Drop Multiplexer (OADM) could be constructed as shown in FIG. 1. In the system of FIG. 1, the OADM 2 is divided into a Drop section 4 and an Add section 6, both of which is constructed around a respective Wavelength-Selective Switch (WSS) 8, 10. In the Drop section 4, the Drop-WSS 8 receives an in-bound Wavelength Division Multiplexed (WDM) signal comprising a set of n parallel wavelength channels from an upstream optical fibre medium 12. The Drop-WSS 8 operates to separate selected channels from the in-bound WDM signal and direct each selected channel to a respective port 14 of the Drop-WSS 8. Consequently, each port 14 outputs a single channel extracted from the in-bound WDM signal. Typically, each port 14 is designated as either a drop port 14d, or a pass-through (or express) port 14e. Each drop port 14d is connected to a respective receiver (Rx) 16 which receives the corresponding optical channel in a conventional manner. Each express port 14e is connected to downstream optical equipment such as, for example, the Add section of the same (or a different) OADM 2. Among other things, this functionality can be used to support branching in an optical mesh network.
The Add section 6 of the OADM 2 operates in a manner that is effectively the reciprocal of the Drop section 4. Thus, the Add-WSS 10 is provided with a set of ports 18, which are designated as either Add-ports 18a or express ports 18e. Each Add-port 18a is connected to a transmitter (Tx) 20, which generates a respective optical channel signal in a conventional manner. Each express port 18e receives a respective optical channel signal from upstream optical equipments such as, for example, the Drop-section 4 of the same (or a different) OADM 2. In each case, the Add-WSS 10 operates to add the channels received through each port 18 into an outbound WDM signal which is launched into a downstream optical fibre medium 22.
One of the problems with the arrangement of FIG. 1 is that conventional WSS components have a limited number of ports 14, 18. Typically, commercially available WSS devices are configured with up to p=9 ports, which must be shared between express ports and drop-ports (in the Drop section) or Add-ports (in the Add-section). WSS components with up to p=20 ports have been demonstrated and may become commercially available in the future. However, increasing the number of ports also tends to increase the cost of the WSS component.
Typical optical transmission systems have between n=32 and n=88 channels. A typical requirement for an OADM node in a network is to be capable of adding/dropping up to 50% of the channels of the WDM signal. In a mesh network, there is also a further requirement for a specified degree of branching to support mesh connectivity. Typically, between 4 and 8 degree branching is required. However, 4-degree branching requires that 3 of the WSS ports be allocated as express ports. In a 9-port WSS component, these leaves only 6 ports available for use as add/drop-ports. If the optical transmission system is designed with an 88 channel capacity, the 6 available add/drop ports represents only a 7% add/drop capacity, which is far below the desired value of 50%.
FIG. 2 is a block diagram schematically illustrating a coherent selection OADM 24 known in the prior art.
As may be seen in FIG. 2, the drop section 26 of a Coherent selection OADM 24 uses a drop-section power splitter 28 to couple an inbound n-channel WDM signal from an upstream optical fibre medium 12 into a plurality of output ports 30. As in the direct attach OADM 2 described above with reference to FIG. 1, the output ports 30 are divided between drop ports 30d and express ports 30e. However, unlike the OADM 2 of FIG. 1, each output port 30 receives energy from all of the channels of the inbound n-channel WDM signal. A coherent receiver (cRx) 32 coupled to each drop port 30d operates to “tune-in” and receive a desired channel from the WDM signal. Because all of the channels of the re-channel WDM signal are output through every port 30 of the drop-section power splitter 28, it is necessary to remove dropped channels from the WDM signal in order to enable channel reuse. This can be achieved by use of a Wavelength Blocker (WB) 34 for 2-connected nodes, or a WSS (not shown) for mesh connected nodes.
The Add section 36 of the OADM 24 operates in a manner that is effectively the reciprocal of the Drop section 26. Thus, an Add-Section power combiner 38 is provided with a set of ports 40, which are designated as either Add-ports 40a or express ports 40e. Each Add-port 40a is connected to a tuneable transmitter (Tx) 42, which generates a respective optical channel signal centered on a desired carrier wavelength, in manner known in the art. Each express port 40e receives a respective WDM optical signal from upstream optical equipments such as, for example, the Drop-section 26 of the same (or a different) OADM 24. In each case, the Add-Section power combiner 38 operates to add the channels received through each port 40 into an outbound WDM signal which is launched into the downstream optical fibre medium 22.
There are two main drawbacks with this approach. The first is increased loss. The large number of drop channels which must be supported drives high port count power splitters and combiners. These devices are used because they are not frequency selective, but as a result have high intrinsic loss. This drives additional cost in amplification and the associated noise increase which limits system performance. This limit eventually limits the number of channels that can be dropped.
The second issue relates to the performance of the coherent transmitters and receivers. In particular, the drop section is inherently non-selective, which means that all of the channels of the inbound WDM signal are presented to each coherent receiver 32. This means that each coherent receiver 32 must be capable of selecting and receiving one channel of interest, while substantially rejecting all of the other channels. The ability of the receiver 32 to perform this function is related to the common mode rejection ratio (CMRR) of the receiver, which drives considerable complexity and cost.
In practice, the additional loss, complexity, and cost of accommodating this solution effectively limit either the number of channels which can be dropped, or the system capacity, or both.
Techniques which overcome at least some of the limitations of the above-noted prior art remain highly desirable.